Denoising diffusion models [1]} trained by optimizing the usual variational bound on negative log likelihood:
\(E\left[ -log(p_{\theta }(x_0) \right] \le E_q\bigg [ -\log p(x_T) - \sum _{t \ge 1} \log \frac{p_\theta (x_{t-1} | x_t)}{q(x_t|x_{t-1})} \bigg ] = L_{VLB}\) 
